Add compatibility layer for extensions relying on old compiler

[GarboMuffin]

The new compiler broke all the extensions doing compiler patches. We promised not to help them when those break but it seems enough people use those extensions that we sort of got forced into finding some solution, so this makes these changes:

• The exports for the new compiler are now in these_broke_before_and_will_break_again
• i_will_not_ask_for_help_when_these_break now returns some stubs that pretend to be the old compiler. It also emits an event so -gui can show a warning.
• Focus is on compatibility & safety, not performance. There will be unnecessary casts and scripts marked as yielding.
• Extensions that meet these assumptions should work without changes:
  • Extensions do not try to combine this compatibility layer with any APIs provided by the new compiler.
  • Extensions treat IR nodes received from descendSubstack and similar as opaque objects because they will be the new intermediate objects instead of { kind: "..." }
  • Extensions need to implement the JS generators for all AST node kinds they use. Can not rely on the default JS generator because those generators are expecting a different format than { kind: "..." }

Lack of tests is intentional since this will be removed at some point when we have a proper API we can tell people to use